We've been talking about anti-skid braking systems in A&P class. They're actually fairly simple: each wheel on the main landing gear has a sensor attached to it that spins with the wheel. Unsurprisingly, they are called wheel speed sensors. They are actually little electric generators that create voltage as the wheels spin. This voltage is used as a signal in a computer. The computer uses that signal to determine when any given wheel is slowing to the point of skidding, and sends a signal to a hydraulic valve to release a bit of brake pressure for that wheel. The goal is to maximize braking power by keeping each wheel right on the edge of skidding.
This is not a trvial feature: the pilots of large, heavy, fast jets cannot feel the wheels beginning to skid, and if the wheels are allowed to skid, they will quickly rip themselves to shreds. When the bare wheel rim hits the concrete of the runway, all braking authority is lost, and with it goes lateral controllability.
I came across a series of photos today that demonstrate exactly why anti-skid is so imporant. Note that I am making the assumption that an anti-skid malfunction was the cause of this incident; I have no actual evidence of that. Regardless, this would be the result of a failed anti-skid system:
Photos copyright by CRASCA, originally posted here: http://s102164210.onlinehome.us/forums/index.php?showtopic=103388&st=0